It's bizarre to say he was suggesting how such as system "could work" as what was suggested turned out to be completely unworkable.As per an earlier post - @Clip was answering a question of *how* a system could work, he wasn't suggesting it was a good idea.
No. It would not result in a "small" increase e.g Gatwick to St Pancras CBA £9.10; I can't see GTR wanting a paper single to be anything less than the PAYG single fare of £8.30, so you're nearly doubling the cost of that return journey.[2] If done on a revenue neutral basis it would result in a small increase to return fares, but I suspect this would indeed be small because most people aren't making single journeys nor complicated ones in a routeing sense.
The TOCs would no longer be able to charge premiums to return another day so the single fare will be based on the more expensive return fares; people who currently buy day returns are going to be heavily penalised under any move to single leg pricing in this country.
The Government could instruct the rail industry to halve the lower priced fares but that will not happen as the TOCs currently make a lot of money out of 'distress purchase' single fares priced at pennies less than a return and the Government are not going to increase subsidy to cover that shortfall. The Government wants rail fares to become less attractive, not more attractive, to reduce the burden on providing additional capacity.
So either it will not go ahead at all (due to negative press of price increases that will NOT be small) or many daytrippers will find their prices have gone up by a huge amount.